I would make a simple cover flap that uses the same attachment points as the pocket, like the Granite Gear lid. It could have some use to carry a rain jacket, sit pad, rolled sleeping pad, etc. it would leave the pack unadulterated and would be much easier to do. A simple rectangle of silnylon and a little hardware would do the trick.

I like the pocket for quick access to items on the trail and can afford the small weight. I keep stuff like my first aid kit, headlamp, snacks, gloves, beanie and sunglasses up there— my glovebox :)